One of the ways in which RNA tumor viruses display their oncogenic potential, is by capturing a cellular oncogene and expressing the oncogene under the control of viral sequences. We are interested in understanding exactly how the capture process occurs and how specific features of the virus life cycle contribute to the process. (i) We will study a special group of viruses, called the recovered Rous sarcoma viruses (rRSV), which have captured the cellular src gene at an accelerated rate. Nucleotide sequence analysis will be used to examine the sites within the viral and cellular genomes that recombined as the initial step in the capture process. (ii) We will develop a new assay that can measure the packaging of cellular RNA sequences into virions, a step that must occur during the capture of cellular genes. Using recombinant DNA methods we will place fragments of viral DNA into a cloned eukaryotic gene and determine which viral fragments can provide a signal that will permit a transcript of the cloned gene to be packaged into virions. (iii) We will map secondary structure near the 5' end of the viral genome in order to understand the organization of the dimer linkage structure, (the site of joining between the two subunits of the dimer viral genome) and the relationship of this structure to other functional units within viral RNA. In this effort we will use a new method that utilizes psoralen as a probe of secondary structure in large RNA molecules. (iv) We will examine the role of a heterologous leader sequences in the expression of src by determining the structure of src mRNA in cells expressing a src-containing fragment of the RSV genome. In a parallel effort we will develop an assay that measures integrative recombination in vitro. This assay will reproduce the reaction that results in the integration of viral DNA into the host chromosome. With this assay we will be able to determine which form of unintegrated viral DNA participates in the integration reaction as well as other features of this process.